U.S. patent number 5,264,097 [Application Number 07/677,525] was granted by the patent office on 1993-11-23 for electrodialytic conversion of complexes and salts of metal cations.
Invention is credited to Daniel J. Vaughan.
United States Patent |
5,264,097 |
Vaughan |
November 23, 1993 |
Electrodialytic conversion of complexes and salts of metal
cations
Abstract
Complexes and salts of metal cations in aqueous solutions are
electrodialytically converted into a solution of acids
substantially free of metal cations and insoluble metal hydroxides.
Metal cations are insolubilized by hydroxyl ions formed in a
catholyte while anions are electrotransported from the catholyte
and converted into a solution of acids by hydrogen ions formed in
an anolyte of an electrodialytic process. The conversion process is
carried out electrically without the electrotransport of metal
cations and is especially useful for reforming solutions of acids
that form complexes when etching and finishing metal surfaces.
Inventors: |
Vaughan; Daniel J. (Wilmington,
DE) |
Family
ID: |
24719068 |
Appl.
No.: |
07/677,525 |
Filed: |
March 29, 1991 |
Current U.S.
Class: |
205/770; 204/539;
205/771 |
Current CPC
Class: |
B01D
61/44 (20130101); C01B 7/0706 (20130101); C25F
7/02 (20130101); C23G 1/36 (20130101); C01B
7/195 (20130101) |
Current International
Class: |
B01D
61/42 (20060101); B01D 61/44 (20060101); C01B
7/19 (20060101); C01B 7/00 (20060101); C01B
7/07 (20060101); C23G 1/00 (20060101); C25F
7/00 (20060101); C25F 7/02 (20060101); C23G
1/36 (20060101); B01D 061/00 () |
Field of
Search: |
;204/151,182.4,301,96,DIG.13 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Niebling; John
Assistant Examiner: Phasge; Arun S.
Claims
I claim:
1. A process for the electrodialytic conversion in an
electrodialytic cell comprising a catholyte and an anolyte
separated by an anion permeable membrane of salts and complexes of
metal cations and anions in an aqueous solution into an insoluble
hydroxide of said metal cations and into acids or halogen of said
salt or complex anions, said aqueous solution being selected from
solutions of said salts or solutions of said complexes and
solutions comprising mixtures of said salts and said complexes
which comprises: including at least one salt of an alkali cation in
said aqueous solution (a) feeding said alkali salt-containing
aqueous solution to said catholyte of said electrodialytic cell,
said catholyte being separated by an anion permeable membrane from
said anolyte of said cell; (b) controlling the pH of said catholyte
by: the rate of removal of anions from said catholyte to said
anolyte, the rate of addition of said aqueous solution to said
catholyte and the rate of forming hydroxyl ions at the cell
cathode, to insolubilize said metal cation as an insoluble
hydroxide in said catholyte and electrotransporting said anions
from said catholyte to convert said anions to acids or to halogen
in said anolyte.
2. The process of claim 1 wherein said catholyte comprises an
alkali cation of potassium or sodium.
3. The process of claim 1 wherein the pH of said catholyte is
controlled at a level greater than the pH necessary to insolubilize
all metal cations that are electrodepositable on the cell
cathode.
4. The process of claim 1 wherein said metal cations are cations of
all multivalent metals and monovalent metals that form insoluble
hydroxides in the pH range of 1 to 14.
5. A process for the electrodialytic conversion in an
electrodialytic cell comprising a first electrolyte, a catholyte
and a second electrolyte of salts or complexes of metal cations and
anions in an aqueous solution into an insoluble hydroxide of said
metal cations and into the acids or halogen of said salt or complex
anions, said aqueous solution being selected from solutions of said
salts or said complexes or solutions of mixtures of said salts and
said complexes which comprises: (a) feeding said aqueous solution
to said first electrolyte of said electrodialytic cell and said
first electrolyte being separated by an anion permeable membrane
from said catholyte of said cell and by an anion permeable membrane
from said second electrolyte in electrical communication with the
anode of said cell; (b) controlling the pH of said first
electrolyte by (1) the rate said aqueous solution is added to said
first electrolyte; (2) the rate acid anions are electrotransported
from said first electrolyte to said second electrolyte; and (3) the
rate hydroxyl ions are electrotransported from said catholyte to
said first electrolyte to insolubilize said metal cations in said
first electrolyte and electrotransporting said acid anions from
said first electrolyte to said second electrolyte to convert said
anions to acids or halogen in said second electrolyte.
6. The process of claim 5 wherein said aqueous solution comprises
multivalent or monovalent metal cations that form insoluble
hydroxides at a pH of two or greater.
7. The process of claim 5 wherein said first electrolyte comprises
an alkali cation of at least one of sodium and potassium.
8. The process of claim 5 wherein said catholyte is an aqueous
solution of an alkali hydroxide of sodium or potassium.
9. The process of claim 5 wherein the pH of said first electrolyte
is controlled at a pH sufficient to insolubilize cations of one
metal and to leave cations of a second metal in solution.
10. The process of claim 5 wherein the pH of said first electrolyte
is sufficiently greater than one to insolubilize all metal cations
in said aqueous solution.
11. A process using an electrodialytic cell for the electrolytic
conversion of salts or complexes of metal cations and anions in an
aqueous solution into an insoluble hydroxide of said metal cation
and the acid of the anions of the salt or complex or halogen if the
anion is a halide which comprises (a) passing an electric current
through said cell having a catholyte separated by an anion
permeable membrane from an anolyte; (b) feeding said aqueous
solution of a complex or salt of a metal cation to said catholyte;
(c) electrotransporting anions from said catholyte to said anolyte;
(d) and controlling the pH of said catholyte to insolubilize said
metal cations in said catholyte and convert anions to acids or
halogen in said anolyte.
12. A process using an electrodialytic cell for the electrolytic
conversion of salts or complexes of metal cations in an aqueous
solution into an insoluble hydroxide of said metal cation and the
acid of the anions of the salt or complex or halogen if the anion
is a halide which comprising (a) passing an electric current
through said cell having a catholyte separated by an anion
permeable membrane from an anolyte; (b) feeding said aqueous
solution to said catholyte; (c) electrotransporting anions from
said catholyte to said anolyte; (d) forming hydroxyl ions in said
catholyte and hydrogen ions in said anolyte whereby the pH of said
catholyte is controlled to insolubilize said metal cations in said
catholyte and convert said anions to acids or halogen in said
anolyte.
13. The process of claim 11 wherein said catholyte comprises a
cation of an alkali metal of potassium or sodium.
14. The process of claim 11 wherein said catholyte comprises a
cation of an alkali metal and preferably a cation of potassium or
sodium.
15. A process using an electrodialytic cell for the electrodialytic
conversion of salts or complexes of metal cations and anions in an
aqueous solution into an insoluble hydroxide of said metal cation
and the acid of the anions of said salt or complex or halogen if
the anion is a halide which comprises (a) passing an electric
current through said cell having at least a catholyte, a first
electrolyte and an anolyte separated by anion permeable membrane;
(b) feeding said aqueous solution to said first electrolyte; (c)
electrotransporting anions from said first electrolyte to said
anolyte to form hydrogen ions in said anolyte and hydroxyl ions in
said catholyte; and (d) electrotransporting hydroxyl ions from said
catholyte to said first electrolyte sufficient to control the pH of
said first electrolyte to insolubilize said metal cation in said
first electrolyte and electrotransporting said anions from said
first electrolyte to said anolyte to convert said anions to acids
or halogen in said anolyte.
16. The process of claim 15 wherein said catholyte is an aqueous
solution of a hydroxide of an alkali cation of potassium or
sodium.
17. The process of claim 15 wherein the pH of said first
electrolyte is controlled sufficiently to insolubilize a cation of
at least one metal.
18. A process using an electrodialysis cell for the electrodialytic
conversion of salts or complexes of metal cations and anions in an
aqueous solution into an insoluble hydroxide of said metal cation
and the acid of the anions of said salt or complex which comprises
(a) passing an electric current through said cell having at least a
catholyte, a first electrolyte and an anolyte said anolyte
separated by a cation permeable membrane from said first
electrolyte and said first electrolyte separated by an anion
permeable membrane from said catholyte; (b) feeding said aqueous
solution to said catholyte to form hydroxyl ions in said catholyte,
(c) electrotransporting anions from said catholyte through said
anion permeable membrane into said first electrolyte to form
hydrogen ions in said anolyte and (d) transporting hydrogen ions
from said anolyte through said cation permeable membrane into said
first electrolyte whereby said metal cation is insolubilized in
said catholyte and said anions are converted to acids in said first
aqueous solution.
19. The process of claim 18 wherein said catholyte contains a
cation of an alkali metal.
20. The process of claim 18 wherein said anolyte is an aqueous
solution comprising an acid of sulfur, nitrogen, phosphorous and
carbon.
21. The process of claim 18 wherein said aqueous solution comprises
salts and complexes of multivalent and monovalent metals that form
insoluble hydroxides at a pH greater than 1.
22. The process of claim 18 wherein the pH of said catholyte is
controlled at a level greater than the pH necessary to insolubilize
all electrodepositable metal cations in said aqueous solution.
Description
FIELD OF THE INVENTION
This invention relates to an electrolytic process for conversion of
complexes of metal cations into insoluble metal hydroxides and a
solution of acids of the complex anions. Specifically this
invention relates to insolubilizing metal cations of complexes and
salts with hydroxyl ions formed in the catholyte of an
electrodialytic process and electrotransporting anions of the
complex or salt from the catholyte through an anion permeable
membrane into an anolyte and conversion of the anions to acids with
hydrogen ions formed at the cell anode. The process comprises
feeding an aqueous solution containing a complex or salt of a metal
cation to a catholyte separated from an anolyte by an anion
permeable membrane and preferably containing an alkali cation
whereby the pH of the catholyte is electrolytically controlled to
insolubilize the metal cation as a hydroxide and anions are
electrotransported into the anolyte and converted into acids. The
instant process does not necessitate the electrotransport of metal
cations through cation permeable membranes and is especially useful
for reforming for use acids and mixtures of acids that form
complexes with metal cations when they are used to etch, pickle and
finish stainless steels, titanium and other metals.
BACKGROUND OF THE INVENTION
Acids are used broadly in the chemical, electronics, mining and
metal finishing industries wherein the acids react with metals to
form salts and complexes. Prior electrodialytic processes, see U.S.
Pat. No. 4,636,288, provide a satisfactory method for reforming
acidic solutions containing multivalent metal salts. These
processes comprise electrotransporting multivalent metal cations
from the acid through a cation permeable membrane and
insolubilizing the metal cation in a catholyte with hydroxyl ions
formed at the cell cathode. Unfortunately, metal cations form
complexes that have no electrical charge or have a negative charge
and the metal cations can not be electrotransported from an acid
through a cation permeable membrane. The electrotransport of metal
cations through cation permeable membranes also present problems in
the prior processes. Multivalent cations form insoluble salts in
membranes or on the surface of membranes that reduce or prevent the
electrotransport of metal cations. At times, cation permeable
membranes are fouled with precipitates when processing acidic
solutions that contain only ppm of a multivalent cation that is not
successfully transported through a cation permeable membrane into a
catholyte. There are few solutions that have no metal cations and
most solutions contain cations of two or more metals which
increases the problems associated with electrotransport of
multivalent metal cations. It would be desirable if acidic
solutions containing metal salts could be reformed into acids of
the salt anions and insoluble hydroxides of the metal cations
without the transport of metal cations through cation permeable
membranes. It would be very desirable to be able to reform
solutions of acids that contain complexes of metal cations in an
electrolytic process without the electrotransport of the
multivalent metal cations. It is an object of this invention to
provide an electrolytic process suitable for reforming acidic
solutions containing complexes and/or salts of metal cations that
does not necessitate electrotransport of multivalent metal
cations.
Electrodialysis is a well-known art (See U.S. Pat. Nos. 4,636,288;
4,325,792; 4,439,293, the disclosures of which are hereby
incorporated by reference.) Electrodialysis is the transport of
ions through ion permeable membranes as the result of an electrical
driving force. The process is commonly carried out in an
electrodialytic cell having an anolyte compartment containing an
anolyte and an anode separated by an ion permeable membrane from a
catholyte compartment containing a catholyte and a cathode. The ion
permeable membrane can be permeable to cations or anions. The anion
permeable membrane usually has fixed positive charges and, as the
names implies, is permeable to anions and relatively impermeable to
cations. The cation permeable membrane usually has fixed negative
charges and is permeable to cations.
There are no cation permeable membranes 100% impermeable to anions
and no anion permeable membranes 100% impermeable to cations. In
all membrane electrodialytic processes there is always some
potential for fouling ion permeable membranes by counter ions
forming precipitates in and on the surface of the membranes.
There are many complexes of metal cations (See Inorganic Chemistry,
Fritz Ephraim, Fifth Edition by R. C. L. Throne and E. R. Roberts )
The complexes can be defined as substances formed by the
combination of components which are already saturated according to
the classical concepts of valency. The coordination number is
commonly six and the complex is not an ion, but is an electrically
neutral compound There are no known groups which definitely confer
a positive charge on a complex of a metal cation There are,
however, substances which can cause an increase in the negative
valency and some complexes can have a negative charge. When metals
are etched, electropolished, bright-dipped or pickled with acids,
such as phosphoric and hydrofluoric, a mixture of complexes and
salts of metal cations are formed in the acid or mixtures of acids.
The increasing need to protect the environment and to conserve
resources make the reuse of the acids and metals desirable. These
acidic solutions usually contain two or more acids, two or more
metal cations and a mixture of complexes and salts of metal cations
and anions of the acids. It is possible to partially reform the
acids using electrodialysis (See U.S. Pat. No. 4,636,288.) by
removing the metal cations of the salts. However, the anions
associated with the metal complex are not reformed and in reuse the
concentration of the metal complex increases in the mixture of
acids and, at some point, the solution of acids must be replaced or
the metal complexes removed. The concentrations of acids and the
level of salts and complexes permissible in the acids vary widely
in the many finishing processes for metals. These complexities
essentially preclude partial reformation of the acids in commercial
processes. It is an object of the present invention to provide a
process suitable for reforming acids and mixtures of acids in
aqueous solutions containing complexes of metal cations or salts of
metal cations and mixtures of complexes and salts of cations of one
or more metals.
SUMMARY OF THE INVENTION
This invention provides an electrodialytic process for conversion
of complexes and salts of metal cations into insoluble metal
hydroxides and acids without the electrotransport of metal cations
through cation permeable membranes. The process comprises (1)
feeding an acidic solution containing a complex or salt of a metal
cation to a catholyte of an electrodialytic process that is
separated by an anion permeable membrane from an anolyte; (2)
adding an alkali cation to the catholyte for controlling pH and
electrical conductivity of the catholyte; (3) electrolysis of water
at the cell cathode to form hydroxyl ions at a controlled pH
whereby metal cations of complexes and salts form insoluble
hydroxides; (4) removing the insoluble metal hydroxides from the
catholyte; and (5) electrotransporting anions from the catholyte
through an anion permeable membrane into an anolyte wherein the
anions are converted into acids by hydrogen ions formed at the cell
anode.
Another aspect of the electrodialytic process of this invention is
the separation of cations of two metals by controlling the pH of
the catholyte whereby the cation of one metal is insolubilized and
the cation of one metal remains in solution. Solutions of nitric
and hydrofluoric acids are reformed from solutions of the acids
used to etch and pickle stainless steels, titanium and other
metals. The process of this invention is broadly applicable for
recovery of acids from solutions containing complexes and salts of
metal cations.
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1A, 1B, and 1CD are schematic representations of the
electrodialytic cells used in the process of this invention;
FIG. 1A showing a two compartment cell; and
FIGS. 1B and 1C showing three compartment cells.
FIG. 2 is a schematic representation of the equipment used in the
process of this invention.
DETAILED DESCRIPTION OF THE INVENTION
The pH at which a complex or salt of a metal cation reacts with
hydroxyl ions in a catholyte to form an insoluble hydroxide of the
metal cation varies with the multivalent metal cation and the acid
anion. In general, the pH in the catholyte is approximately the pH
required to insolubilize the metal cation and to form an alkali
salt of the anion when an alkali hydroxide is added to an acidic
solution containing a metal salt or complex. For example, the pH at
which some metal cations begin to precipitate in aqueous solutions
is: Mg 10.5, Mn.sup.++ 8.8, Nd 7.0, Zn.sup.++ 7.1, Co.sup.++ 6.8,
Ni.sup.+ 6.7, Pb.sup.++ 6.7, Fe.sup.++ 5.7, Cu.sup.++ 5.5,
Cr.sup.+++ 5.3, Al.sup.+++ 4.1, Sn.sup.++ 2.0, Fe.sup.+++ 2.0,
Ti.sup.++++ 2.0. When the pH is equal or higher than the pH where
metal cations form precipitates, complexes and salts of metals are
converted into insoluble metal hydroxides and soluble ionically
mobile anions. By controlling the pH at a level to insolubilize a
cation of one metal and leave a cation of another metal in
solution, it is possible to separate cations of two metals that
form insoluble hydroxides at a different pH. When it is desirable
to insolubilize all metal cations, the pH must be equal to or
higher than that required for the metal cation that precipitates at
the highest pH.
The electrical current flowing through an electrochemical cell is
carried by ions and is effected by the electron transfer reactions
at the electrodes which upset the equivalence of positive and
negative ions. Oxidation reactions occur at the cell anode and
reduction reactions occur at the cell cathode For example, water is
oxidized to hydrogen ions and oxygen gas at the cell anode and
water is reduced to hydroxyl ions and hydrogen gas at the cell
cathode. The hydrogen ions migrate toward the negatively charged
cell cathode and the hydroxyl ions migrate toward the positively
charged anode. There is essentially ion neutrality in all
electrolytes of an electrodialytic process. When the hydrogen ion
concentration equals the hydroxide ion concentration, as it does in
pure water, the electrolyte is said to be neutral and electrically
non-conductive Therefore, the electrolytes must contain equal
concentrations of negatively charged ions (anions) and positively
charged ions (cations). The cations can be hydrogen ions or metal
cations and the anions can be hydroxyl ions or anions of acids.
When hydroxyl ions replace acid anions of salts, water is formed
and when hydroxyl ions react with hydrogen ions, water is formed.
When a hydrogen ion replaces a metal cation of a salt, an acid is
formed. When a hydroxyl ion reacts with hydrogen ions, water is
formed and with metal cations, a hydroxide is formed. It is safe to
assume that essentially all hydrogen ions in a solution come from a
dissolved acid and that all hydroxyl ions come from a dissolved
base and that the dissociation of water is negligible. In
electrochemical cells divided by ion permeable membranes it is
possible to effect various separations of electrolytes between the
anode and cathode of the cell. For example, in a cell having an
anolyte compartment and a catholyte compartment separated by a
cation permeable membrane, hydrogen ions formed at the cell anode
replace cations of salts to form acids in the anolyte and the
replaced cations and hydrogen ions formed at the anode are
electrotransported through the cation permeable membrane into the
catholyte. The hydrogen ions react with hydroxyl ions in the
catholyte to form water and the cations to form soluble and
insoluble hydroxides. For example, in a two compartment cell with
the compartments separated by an anion permeable membrane, hydroxyl
ions and anions of acids could be electrotransported through the
anion permeable membrane from the catholyte to the anolyte where
the hydroxyl ions would react with hydrogen ions at the anode to
form water and the anions of acids would react with hydrogen ions
to form acids.
Therefore, I thought it should be possible to feed an acidic
solution containing metal complexes and salts to the catholyte of
an electrodialytic cell, the catholyte being separated by an anion
permeable membrane from the anolyte of the cell, and passing an
electric current through the cell would cause metal cations of
salts and complexes to be precipitated as insoluble hydroxides and
the anions of the acids electrotransported to the anolyte and
converted to acids as shown in FIG. 1A. I found that the process
could be used. However, the pH of the catholyte was limited to
seven or less. Removal of the acid anions and precipitation of the
metal cations made the catholyte limiting in electrical
conductivity. Some metal cations of complexes and salts are not
precipitated at a pH of seven or less and they accumulated in the
catholyte. At a pH less than seven when two metal cations were fed
to the catholyte, one would precipitate and the other remain
soluble or be electrodeposited on the cathode. Attempts to
eliminate electrodeposition on the cathode by using a three
compartment cell and feeding the acidic solution with metal salts
and complexes to the center compartment were only partially
successful. The anion membrane separating the center feed
compartment became fouled with metal salts and hydroxides and there
was some deposit of metal on the feed side of the membrane. The
limitations of electrical conductivity, limited pH and attendant
problems in the catholyte were not changed by adding the feed to
the center compartment and using a dilute solution of alkali
hydroxide in the catholyte compartment.
I have now found that the addition of an alkali cation to the
catholyte or to the center feed compartment provides a simple
solution to electrical conductivity, limits of pH, complete
insolubilization of metal cations and reduced capacity and
efficiency. The alkali cation forms a soluble salt with acid anions
and a soluble base with hydroxyl ions. The pH can range up to a pH
of 14. The hydroxyl ions formed at the cell cathode can associate
with the alkali cation to form a soluble hydroxide instead of
hydrogen ions to form water. This instant process is broadly
applicable for conversion of complexes and salts of metal cations
to insoluble hydroxides of the metal cations and acids of the
anions. This is accomplished without the electrotransport of metal
cations through cation permeable membranes and the fouling of anion
permeable membranes with metals, metal salts and insoluble metal
hydroxides.
The electrodialytic process of this invention can be carried out in
electrochemical cells having two or more compartments. A two
compartment cell as shown in FIG. 1A has an anolyte and a catholyte
compartment separated by an anion permeable membrane. A solution
containing acids and metal salts or complexes is fed to the
catholyte. The pH of the catholyte is controlled whereby hydroxyl
ions formed at the cathode react with the metal cations to form an
insoluble metal hydroxide and anions are removed from the catholyte
through the anion permeable membrane into the anolyte and converted
to acids by hydrogen ions formed at the anode. The insoluble metal
hydroxide is removed from the catholyte by filtration or other
methods for separation of solids and liquids. The concentration of
the acids is controlled by addition of water to the anolyte. In a
two compartment cell, it is preferable to control the pH of the
catholyte to insolubilize all metal cations that are
electrodepositable on the cathode.
The pH of the catholyte is controlled by the rate the acidic
solution containing a complex or salt of a metal cation is added to
the catholyte, the rate of removal of anions of acids from the
catholyte and the rate of formation of usable hydroxyl ions in the
catholyte. Anions of acids that are removed from the catholyte are
replaced by hydroxyl ions formed at the cell cathode. Metal cations
and hydrogen ions added to the catholyte form water and insoluble
hydroxides with hydroxyl ions in the catholyte. Hydroxyl ions and
acid anions removed from the catholyte to the anolyte form water
and acids respectively with hydrogen ions formed at the cell anode.
For operation over the range of pH of about 2 to about 10, normally
required for insolubilization of metal cations, it is preferable to
add an alkali cation to the catholyte for adjusting and controlling
the pH of the catholyte and removing metal hydroxides from the
catholyte. The alkali cation reacts with hydroxyl ions to form an
alkali hydroxide and acid anions to form soluble salts. The level
of alkali cations can be varied over a wide range to affect
electrical conductivity of the catholyte and formation of a soluble
hydroxide for reaction with metal cations. A preferred method for
operation of the instant process is shown schematically in FIG. 2.
The acidic feed with a complex or salt of a metal cation is added
to Tank 1 containing the catholyte at a pH to insolubilize a metal
cation. The insoluble metal hydroxide is separated from the
catholyte by filtration or other means of separating solids from
liquids. The filtrate is passed through the catholyte compartment
of the electrolytic cell where anions are removed and alkali
hydroxide formed and then returned to Tank 1 for reaction with
additional feed solution. The pH of the catholyte is preferably
maintained at a value to insolubilize in Tank 1 all metal cations
in the feed solution.
The electrodialytic process of this invention can be carried out in
cells that have more than two compartments as illustrated in FIG.
1B and 1C. In FIG. 1B, the compartments of the cell are separated
by anion permeable membranes. The catholyte compartment 1 contains
an alkali hydroxide catholyte. Compartment 2 is the feed
compartment which has an electrolyte containing an alkali cation
and having a controlled pH and compartment 3 is the anolyte
compartment. The process as shown in FIG. 1B precludes
electrodeposition of metals on the cell cathode when it is desired
to separate cations of two or more metals. The feed compartment and
feed electrolyte are preferably operated as described for the
catholyte in FIG. 2. Hydroxyl ions formed at the cell cathode are
electrotransported from the catholyte to the feed electrolyte.
When the feed solution contains fluoride ions that dissolve anodes
and chloride ions that will be oxidized to chlorine at the cell
anode, it is preferable to separate the anode and anolyte from
these anions by a cation or bipolar membrane as shown in FIG. 1C.
The anolyte is preferably a dilute solution of an acid without
halides. Hydrogen ions are electrotransported through the cation
permeable membrane into the anion receiving electrolyte to effect
ion neutrality of the electrodialytic process. The process as shown
in FIG. 1C could be carried out in a four compartment cell with the
catholyte being a solution of alkali hydroxide.
Cells having more than three compartments can be used in the
process of this invention provided that the catholyte compartment
is separated by an anion permeable membrane from another cell
compartment receiving hydroxyl ions from the cell cathode to
insolubilize a metal cation.
The feed to the electrodialytic process of this invention is any
aqueous solution that contains a complex or a salt of a metal
cation that can be insolubilized by hydroxyl ions and an anion that
can be electrotransported through an anion permeable membrane and
converted into an acid. The feed solution may contain cations of
two or more metals and anions of two or more acids. The feed
solution may contain only salts of metal cations or only complexes
of metal cations or mixtures of salts and complexes. The feed
solution may contain salts and complexes of monovalent metal
cations, i.e., silver, nickel, copper, that form insoluble
hydroxides and alkali cations that form soluble salts.
The pH of the catholyte or feed electrolyte, if not the catholyte,
can vary from a pH of fourteen to a pH of about 2. It is preferable
that the pH of the catholyte be adjusted to insolubilize all metal
cations that are electrodepositable on the cell cathode and that
the catholyte or feed electrolyte contains sufficient alkali
cations to maintain a desired pH and electrical conductivity. The
concentration of alkali cations can be varied over a wide range to
facilitate adjustment and maintenance of pH and electrical
conductivity of the feed electrolyte.
The catholyte can be the feed electrolyte or a catholyte supplying
hydroxyl ions to a feed electrolyte to insolubilize a metal cation
in the feed electrolyte. The catholyte may be a solution of an
alkali hydroxide with or without an alkali salt. When the catholyte
is a solution of an alkali hydroxide, it is preferable that the
concentration of alkali hydroxide be sufficiently low to preclude
degradation of the anion membrane in contact with the
catholyte.
The anolyte is preferably an aqueous solution of acids of anions in
the feed solution except when the feed solution contains fluoride
and chloride ions and then it is preferable that the anolyte be a
dilute solution of an acid separated by a cation permeable membrane
from an electrolyte containing chloride and fluoride.
Any anion permeable membrane can be used to separate the
compartments of the electrodialytic cell. These anion permeable
membranes have fixed positive charges distributed in the membrane
matrix and are relatively impermeable to cations. The membranes are
preferably membranes of hydrocarbon and halocarbon polymers
containing quaternary ammonium or tertiary amine groups. The
preferred membranes are substantially chemically stable to the
process conditions and mechanically suitable for design and
economical operation of the electrodialytic process. Suitable
membranes are Ionac.RTM. MA-3475 from Sybron Chemicals, Inc. and
TOSFLEX.RTM. IE-SF34 fluorinated anion membrane from TOSOH
Corporation. The preferred membranes for strong caustic and
oxidizing media are the perfluorinated membranes. Any cation
permeable membrane can be used to separate the anolyte compartment
from other cell compartments. These cation permeable membranes have
fixed negative charges distributed in the polymer matrix and are
permeable to positively charged ions. The membranes are preferably
membranes of hydrocarbon and halocarbon polymers containing acids
and acid derivatives. The preferred membranes are substantially
chemically stable to the process conditions and mechanically
suitable for design and economical operation of the electrodialytic
process. Perfluorocarbon membranes such as NAFION.RTM.,
manufactured by Dupont, are preferred for strong oxidizing
environments and temperatures above 80.degree. C.
The alkali cations of the process of this invention can be a cation
of an alkali metal or ammonium and preferably a cation of potassium
or sodium.
Cathodes for the process may be any electrically conductive
material resistant to the catholyte. Such materials are iron,
stainless, steel, nickel, titanium with nickel coatings, reduced
oxides of titanium and the like. While solid cathodes may be used,
foraminous cathodes are preferred.
Anodes for the process of this invention may be any electrically
conductive, electrolytically active material resistant to the
anolyte. Materials such as a value metal of titanium, tantalum or
alloys thereof bearing on its surface a noble metal, a noble metal
oxide either alone or in combination with a value metal oxide, lead
dioxide or other electrolytically active materials are generally
preferred. The anodes may be of a ceramic of reduce oxides of
titanium such as Ebonex.RTM. from Ebonex Technologies. The anodes
may be solid but foraminous anodes are generally preferred for
release of gas and higher surface areas.
To illustrate the practice of the instant invention, three
electrodialytic cells were assembled, as illustrated in FIG. 1,
having a different number of compartments separated by ion
permeable membranes. The electrolysis area based on the area of one
membrane surface in contact with electrolytes was about 45 sq. cm.
or 7 sq. inches. The cell was equipped for circulating the feed
electrolyte to a holding tank and back to the cell compartment to
effect removal of solids, addition of feed solution and measuring
and adjusting pH of the feed electrolyte. Electrical power was
supplied by a Hewlett Packard power supply equipped for operation
at a fixed voltage and variable current or variable voltage and
fixed current. Provisions were made for sampling all electrolytes
and controlling the respective volumes of the electrolytes. Each
experiment was run for about two hours to ensure that metal cations
in the feed solution were insolubilized and anions were removed
from the feed electrolyte and converted into their respective
acids.
Most of the feed solutions were obtained from companies using the
solutions commercially. No attempt was made to identify all metal
cations. The filtrate of the feed electrolyte was tested for
soluble metal cations and anions. The filtrate was tested for the
major multivalent metal cations. The acidity of the anolyte or
electrolyte receiving electrotransported anions was determined as a
measure of removal of anions from the feed electrolyte. In
experiments to illustrate the separation of metal cations at a
controlled pH, the filtrate of the feed electrolyte and the solids
were tested for metal cations. This invention is for the
insolubilization of multivalent metal cations in complexes and
salts by hydroxyl ions formed at the cell cathode. Operation of the
process is readily apparent by the formation of solids in the feed
electrolyte.
EXAMPLE 1
A two compartment cell as illustrated in FIG. 1A was assembled. The
cell compartments were separated by an Ionac.RTM. MA-3475 anion
permeable membrane. The cathode was a mesh of titanium coated with
nickel and the anode was a mesh of titanium coated with iridium
oxide. The catholyte compartment was connected to a surge tank
equipped with a pH sensor, conduits for circulation of the
catholyte through the catholyte cell compartment and for adding
feed and removal of solids. The anolyte compartment was equipped
for removal of anolyte. The volume of the anolyte compartment was
200 ml. and the catholyte system 2,000 ml. Electrolysis was carried
out at 15 amperes and variable voltage. The catholyte system was
filled with a 2 wt. % solution of sodium hydroxide and the anolyte
compartment with a 0.5 wt. % solution of sulfuric acid. An
electropolishing solution containing 20 wt. % sulfuric acid, 15 wt.
% glycolic acid and about 3 wt. % of dissolved metal from polishing
stainless steel was fed to the catholyte solution to reduce the
catholyte pH from 14 to 7.5 and then at a rate to maintain the
catholyte at a pH ranging from 7.5 to 7.0. Metal hydroxides were
formed continuously and removed by filtration of the catholyte. The
acidity of the anolyte increased continuously and was maintained at
a 20 wt. % solution of acid, calculated as sulfuric acid. After
five hours of operation the feed was stopped and the solutions
removed from the cell compartments. The catholyte solution was
water white. There was no significant deposit on the cell cathode.
The electrical efficiency for removal of acid anions from the
catholyte compartment was about 85%. There were no solids that
separated from the catholyte when the pH of the catholyte was
increased to 12.5.
The cell was cleaned with water, the catholyte system filled with a
1 wt. % solution of sodium hydroxide, the anolyte compartment
filled with a 5 wt. % solution of citric acid, and electrolysis
initiated at 15 amperes. A solution containing 55 wt. % citric
acid, 15 wt. % sulfuric acid and about 10 wt. % of dissolved metals
consisting of iron, copper, nickel and zinc was added to the
catholyte and the pH adjusted to 6.5. Solids were formed
continuously in the catholyte and removed by filtration. At a pH of
6.5, the catholyte was a very pale green indicating that a metal
complex was not completely converted into a metal hydroxide. The pH
was increased to 8 by reducing the feed rate. The catholyte at a pH
of 8 was water white indicating essentially complete breaking of
the metal complex. The acidity of the anolyte increased
continuously and was maintained at 22 wt. % acid, calculated as
sulfuric acid. The ratio of citric to sulfuric was close to the
ratio of the two acids in the feed. After four hours, the feed was
stopped and the anolyte and catholyte removed from the cell.
These examples show the insolubilization of metal cations and the
reforming of acids of the anions by feeding an acidic solution to a
catholyte containing alkali cations at a rate to maintain a pH of
the catholyte high enough to break metal complexes and insolubilize
the metal cations with hydroxyl ions formed at the cell cathode.
The process is carried out with electricity and electrolysis of
water at the cell cathode and cell anode. The alkali cations remain
in the catholyte and the multivalent metal cations are removed as
solids.
EXAMPLE 2
A three compartment cell as shown in FIG. 1B was assembled by
adding a compartment between the catholyte compartment and anolyte
compartment of the two compartment cell used in Example 1. The
third or center compartment was separated from the catholyte
compartment by an Ionac.RTM. MA 3475 anion permeable membrane and
from the anolyte compartment by a NAFION.RTM. 417 perfluorinated
membrane permeable to cations. The cathode was made of rods of 304
stainless steel and the anode was a mesh of titanium coated with
platinum and iridium. The electrode gap between anode and cathode
was 0.64 cm. The center compartment was equipped with conduits for
fluid flow of electrolyte and a surge tank for measuring pH and
addition of solutions (center compartment system). The catholyte
system, as described in Example 1, was filled with a 2 wt. %
solution of potassium hydroxide, the center compartment system with
a 0.5 wt. % solution of phosphoric acid and the anolyte compartment
was filled with a 0.5 wt. % solution of phosphoric acid. Current
was passed through the electrodialytic cell at 15 amperes and a
solution comprising 25 wt. % phosphoric acid, 3 wt. % hydrochloric
acid and 0.8 wt. % nitric acid containing salts and complexes of
nickel, chromium.sup.+ and iron. The solution, supplied by
Stainless Micro-polish Inc., Anaheim, Calif., used for
electropolishing, was fed to the catholyte system at a rate to
maintain a pH of 8.5. Metal hydroxides were formed continuously in
the catholyte and removed by filtration. The catholyte was water
white. The acidity of the center compartment electrolyte increased
continuously to a 10 wt. % acidity calculated as phosphoric acid.
The ratio of phosphoric acid to hydrochloric acid was approximately
nine. The acidity of the anolyte remained essentially 0.5 wt. % at
initial volume, water was added to maintain volume of the anolyte.
Oxygen from the anolyte was scrubbed in a 5 wt. % solution of
sodium hydroxide and the scrubbing solution analyzed for
hypochlorite. Only traces of hypochlorite were detected after five
hours of operation.
The feed rate was increased and adjusted to a pH of 6. The
catholyte solution slowly developed a light pale blue-green color
indicating that a metal complex was not being completely converted
to a metal hydroxide. The electrolysis was terminated after three
hours and the electrolytes removed from all cell compartments.
The cell compartments were filled as follows: anolyte, 0.5 wt. %
nitric acid; center, 0.5 wt. % nitric acid; and catholyte, 2 wt. %
sodium hydroxide. Electrolysis was started and maintained at 15
amperes while a solution of 30 volume % of nitric acid, 4 volume %
hydrofluoric acid containing 2.5 wt. % metal complexes and salts
formed in pickling stainless steel was fed to the catholyte at a
rate to control the pH of the catholyte at 9.5. Metal hydroxides
were formed continuously in the catholyte and were removed by
filtration. The catholyte solution remained essentially colorless
and the acidity of the center compartment increased to 10 wt. %,
calculated as nitric acid. The ratio of nitric and hydrofluoric was
approximately that in the feed. After three hours, the feed rate
was increased to maintain the catholyte at a pH of eight. Metal
hydroxides were continuously formed in the catholyte but the
catholyte had a very light green color indicating that a metal
cation was not precipitated at a pH of eight. The cathode remained
essentially free of deposits. Electrolysis was terminated after
three hours and all solutions removed from the cell
compartments.
The cell compartments were filled with solutions as follows:
anolyte, 1 wt. % sulfuric acid; center, 1 wt. % hydrochloric;
catholyte, 2 wt. % sodium hydroxide. Electrolysis was started and
maintained at 15 amperes while a solution containing cuprous
chloride, cupric chloride and hydrochloric acid, an etchant for
copper, was fed to the catholyte at a rate to maintain a pH of 8.5.
Cuprous and cupric hydroxides were formed continuously and filtered
from the catholyte. The catholyte was water white and there was no
detectable deposit of copper on the cathode. The acidity of the
center compartment electrolyte increased to a 10 wt. % solution of
HCl and maintained at this value. The oxygen from the anolyte
contained traces of chlorine. After five hours, the electrolysis
was terminated and solutions removed from the cell
compartments.
The NAFION.RTM. 417 membrane separating the anolyte compartment and
center compartment was replaced with an anion permeable membrane
TOSFLEX.RTM. IE-SF34 made by TOSOH Corporation. The following
electrolytes were added to the compartments: anolyte, 0.5 wt. %
phosphoric acid; center compartment, 2.0 wt. % sodium hydroxide;
catholyte, 1.0 wt. % sodium hydroxide. Electrolysis was controlled
at 15 amperes while a solution comprising 80 wt. % orthophosphoric
acid, 3 wt. % nitric acid and 35 g/l of dissolved aluminum with
copper alloy was fed to the center compartment at a rate to
maintain the pH of the center compartment electrolyte at 8.0.
Aluminum, copper and other metals in the alloy of aluminum were
precipitated continuously at a pH of 8 and removed from the
electrolyte by filtration. The electrolyte was essentially free of
color. The acidity of the anolyte increased to 10 wt. % and was
maintained at 10% by addition of water. The catholyte solution
remained water white at essentially 1.0 wt. % sodium hydroxide. The
volume of catholyte was maintained by addition of water.
EXAMPLE 3
A four compartment cell was assembled by adding a compartment
between the anolyte compartment and the center compartment of the
cell used in Example 2. The cell compartments were numbered from
the anolyte compartment being number 1 and the catholyte
compartment no. 4. The anolyte compartment was separated from
compartment no. 2 by a NAFION.RTM. 417 perfluorosulfonic membrane
that was permeable to cations. Compartment no. 2 was separated from
compartment no. 3 by an anion permeable membrane, TOSFLEX.RTM.
IE-DF34, and compartment no. 3 was separated from the catholyte
compartment by an Ionac.RTM. MA 3475 anion permeable membrane. The
cell compartments were filled with the following solutions: anolyte
compartment no. 1, a 0.5 wt. % nitric acid; compartment no. 2, a
0.5 wt. % nitric acid; compartment no. 3, a 2 wt. % solution of
sodium hydroxide; and compartment no. 4, catholyte, a 2 wt. %
sodium hydroxide. While electricity was passed through the cell, a
nitric acid-hydrofluoric acid solution used for pickling titanium
containing about 30 wt. % nitric acid, and 4 wt. % hydrofluoric
acid and 30 g/l of dissolved titanium was fed to compartment three
at a rate to maintain the electrolyte at a pH of 3. Titanium
cations were insolubilized continuously and removed from the
electrolyte in compartment no. 3. The catholyte remained water
white and the concentration of nitric and hydrofluoric acid
increased in the no. 2 electrolyte. Traces of hydrofluoric acid
migrated to the anolyte without measurable damage to the anode over
a period of five hours of electrolysis. The electrolysis was
terminated and all solutions removed from the cell
compartments.
The cell compartments were again filled with solutions as follows:
anolyte compartment no. 1, a 1 wt. % sulfuric acid; compartment no.
2, a 2 wt. % solution of hydrochloric acid; compartment no. 3, a 2
wt. % of sodium hydroxide; and compartment no. 4, catholyte, a 2
wt. % solution of sodium hydroxide. While electricity was passed
through the cell at 15 amperes, a solution of ferric chloride
containing 8 grams per liter of dissolved copper was fed to the no.
3 compartment electrolyte at a rate to maintain the electrolyte at
a pH of 2.5. Ferric hydroxide was precipitated continuously as a
rusty hydroxide and the catholyte when filtered was a blue color
with some green. An aliquote of the catholyte was removed, analyzed
and found to contain copper and ferrous iron. Increasing the pH of
the electrolyte from 2.5 to 6.5 resulted in precipitation of the
copper and ferrous cations and the catholyte was water white. After
four hours the electrolysis was terminated and the electrolytes
removed from the cells. The catholyte remained at a 1 wt. %
solution of sodium hydroxide with water addition to maintain
volume. Traces of chlorine were detected in the oxygen from the
anolyte.
These examples show the insolubilization of metal cations in
complexes and salts in the catholyte containing an alkali cation of
an electrodialytic cell and in an electrolyte containing an alkali
cation in a compartment separated from the catholyte by an anion
permeable membrane. Also, these examples show the separation of
anions from the feed electrolyte and conversion of the anions to
acids in a compartment separated from the feed compartment by an
anion permeable membrane. The use of a cation permeable membrane to
separate chloride ions to minimize formation of chlorine and
fluoride ions from migrating to the anolyte and dissolving the
anode are also demonstrated. A large number of solutions containing
metal complexes and salts have been processed successfully and the
metal cations precipitated as metal hydroxide and the anions
converted to acids. Addition of the feed solution to the catholyte
is advantageous in efficiency of precipitation of the metal cations
and facilitates maintaining a water balance. Feeding the solution
containing metal complexes or metal salts to the center compartment
electrolyte containing an alkali cation provides for separation of
metal cations without electrodeposition of metal on the cell
cathode.
A large number of solutions containing metal complexes or salts and
mixtures of salts and complexes were successfully processed as
illustrated by these examples. The insolubilization of metal
cations with hydroxyl ions formed at the cell cathode makes
possible the conversion of metal complexes and metal salts into
hydroxides of the metal cations and acids of the anions of acids
without electrotransport of metal cations through cation permeable
membranes. The process of the instant invention is especially
useful in conversion of metal complexes to acids of the anions of
the complexes and insoluble hydroxides of the metal cations of the
complexes.
* * * * *